When a flow rate of fluid is to be controlled, a plurality of fluid control instruments are provided separately in a channel so as to constitute a flow rate control device. For example, a pressure-type flow rate control device that controls the flow rate of fluid with a sonic nozzle includes a fluid control valve, a pressure sensor, and a restriction mechanism serving as a fluid resistance, which are provided separately in this order from the upstream side in a channel. In such a pressure-type flow rate control device, the opening of the fluid control valve is controlled on the basis of a measured value of the pressure sensor such that pressures across the restriction mechanism serving as the fluid resistance are at the critical pressure ratio or lower.
A restriction mechanism used in such a pressure-type flow rate control device may include, aside from a restriction, a valve body and an actuator for actuating the valve body. A restriction mechanism disclosed in Patent Literature 1 includes a restriction block that includes an inner channel provided with an orifice and a needle serving as a valve body for regulating the inner diameter of a constricted portion of the orifice. Meanwhile, a restriction mechanism disclosed in Patent Literature 2 includes a main body block provided with an inner channel, a nozzle block mounted so as to fit in the inner channel, and a valve seat that is a valve body capable of making or breaking contact with a downstream-side opening of the nozzle block and that prevents gas from the downstream side from flowing backward when being closed.
In the above-described flow rate control device, the fluid control valve, the pressure sensor, and the restriction mechanism are provided separately from one another in the channel, which requires an internal volume in the flow rate control device corresponding to the space for a channel for connecting the respective instruments. As the internal volume inside the flow rate control device increases, so does the amount of fluid that remains in the internal volume when, for example, the valve is fully closed so as not to flow the fluid. Consequently, it takes more time for the flow rate of the fluid to become substantially zero after the valve is fully closed.
Therefore, in consideration of improving the falling response performance of the flow rate control device, the instruments are conventionally disposed as close as possible to one another so that the channel for connecting the instruments can be made narrow and short in order to reduce the internal volume.
However, if the channel for connecting the respective instruments is too narrow and too short, there arises a possibility that an unintended pressure loss occurs at a portion other than the fluid resistance, which may deteriorate an accuracy in the flow rate control, for example. Thus, it has been difficult to form a channel within a certain size of an internal volume.
And if the internal volume inside the fluid control valve or the restriction mechanism constituting the flow rate control device is reduced in order to improve the falling response performance, for example, a similar problem to the one described above may occur. In particular, because such restriction mechanism is not constructed for the purpose of reducing the internal volume, an extreme reduction of the internal volume may destroy a function as the restriction mechanism to a large extent.